In recent years, high-efficiency transfer with digitalization has been employed widely in wireless communication. When a multi-value phase modulation system is applied to wireless communication as a method of executing high-efficiency transfer, widely performed is a technique in which a transmission side, in particular, linearizes an input/output characteristic of an amplifier, and suppresses non-linear distortion so as to reduce leakage power to adjacent channels. When power efficiency is tried to be improved while using an amplifier having deteriorated linearity, non-linear distortion that is generated by the amplifier is preferably compensated.
In mobile communication such as the wideband code division multiple access (W-CDMA), transmission power of a transmission apparatus is ten mW to several tens W and the input/output characteristic of a power amplifier has non-linearity. The input/output characteristic of the power amplifier in this case is expressed by a distortion function f(p). This non-linear characteristic generates non-linear distortion and a frequency spectrum around a transmission frequency f(0) has a raised side lobe. This causes leakage of the power to the adjacent channels, resulting in adjacent interference. That is, the non-linear distortion increases electric power of a transmission wave that leaks to the adjacent frequency channels. The magnitude of the leakage power is expressed by an adjacent channel power ratio (ACPR) as a ratio between electric power in the channel of the own apparatus at a frequency centered at f(0) and adjacent leakage power leaked to the adjacent channels. This indicates that when the adjacent leakage power is larger than the electric power in the channel of the own apparatus, the leakage power is considered to be increased. The leakage power affects other channels as noise and deteriorates communication quality of the other channels affected by the leakage power.
The leakage power is small on a linear region on the input/output characteristic of the power amplifier, for example, and is large on a non-linear region. In order to increase output of the power amplifier, it is considered that the linear region on the input/output characteristic of the power amplifier is made larger. Although it is considered that an amplifier having a high capacity is used in order to make the linear region larger, cost is increased and the apparatus is increased in size.
For solving this problem, a distortion compensation device employing a digital linearity distortion compensation system is provided on a wireless communication apparatus in some cases in order to suppress distortion of a transmission signal that has passed through the amplifier. The digital linearity distortion compensation system is a distortion compensation system of detecting a carrier wave obtained by quadrature modulation with a modulation signal in a feedback manner, digital-converting amplitudes of the modulation signal (transmission baseband signal) and a feedback signal (feedback baseband signal) to compare these signals, and updating a distortion compensation coefficient based on the comparison result sequentially. For example, the distortion compensation device employing the digital linearity distortion compensation system obtains an inverse characteristic of an amplification characteristic by using the distortion compensation coefficient. Then, the distortion compensation device employing the digital linearity distortion compensation system applies distortion of the inverse characteristic of the amplification characteristic to the transmission signal before being input to the amplifier. The application of the distortion of the inverse characteristic suppresses distortion of the transmission signal that has passed through the amplifier, so that non-linearity of the amplifier is compensated.
As the distortion compensation device, there is a distortion compensation device that stores a plurality of distortion compensation coefficients to be used for distortion compensation in a lookup table (LUT) and reads the distortion compensation coefficient from the LUT by specifying an address in accordance with a power value of a transmission signal, for example. The distortion compensation device generates an LUT address for compensating non-linearity of the amplifier in accordance with the power value, the amplitude, or the phase of the signal, for example.
It is known that a phenomenon of a memory effect on the amplifier occurs. The memory effect is an effect that the output of the amplifier at a certain time is affected by input in the past due to an electric transient response called Idsp drift. If the memory effect of the amplifier is generated, it is difficult for the distortion compensation device that generates only the LUT address in accordance with the power value of the signal at the current time to maintain distortion compensation performance.
For solving this, proposed has been a technique of generating a second LUT address in consideration of variation of power values of signals in the past in addition to a first LUT address in accordance with the power value of the signal at the current time. In the technique, the second LUT address is generated by using a simple moving average value or a weighted moving average value obtained by averaging the power value of the signal at the current time and the power values of the signals in the past and the distortion compensation coefficient is read from the LUT by using the first and second LUT addresses. This reproduces non-linearity of the amplifier in accordance with an instantaneous value of the electric power of the signal by using the distortion compensation coefficient with high accuracy and suppresses the generation of the memory effect due to the electric transient response on the amplifier. This makes it possible to maintain distortion compensation performance.
Patent Document 1: Japanese Laid-open Patent Publication No. 2011-199428.
The related techniques do not consider to suppress increase in a circuit size while maintaining distortion compensation performance. To be more specific, in the related technique of generating the LUT address by using the simple moving average value or the weighted moving average value, a plurality of sample values obtained by delaying the current signal temporally are generated on the assumption of calculation of the simple moving average value or the weighted moving average value. That is, a plurality of delay circuits are used in the related technique. This arises a risk of an increased circuit size. Furthermore, the related technique uses a plurality of memories for holding a plurality of weighting coefficients to calculate the weighted moving average value. This also increases the circuit size.